I'm trying to figure out whether it's okay to go from a #14 wire NM to #12 NM when extending an existing circuit. (OCPD is 15a) I've tried voltage drop; I've tried Ohms Law; but neither theory is helping. I can't find anything against it, however. And as far as I can tell, as long as the circuit remains protected at 15a everything should be fine.

I take it you are referring to 250.122(B) for the EGC upsizing. I'm trying to understand the technical reasons for this.

First, 250.122(B) excludes ampacity adjustment factors. But it seems to me that when it is necessary to the EGC be sized for no less than the OCPD rating, a condition that would make that OCPD related sizing crucial (e.g. some fault condition is pulling full load current through the EGC), the thermal aspects in the raceway would still be effectively the same. I don't understand why this exception exists.

However, in the case of a long circuit being upsized only for a voltage drop condition, I don't see an issue like that. But maybe it could be the case that a fault to EGC could fail to open the OCPD where the circuit length has to much resistance?

Of course there is the concern that if upsized conductors are present at the circuit origination, some handyman might think he can upsize the OCPD. That risk is also present if the branch circuit changes to a smaller size at a later point.

What about cases where the neutral is oversized for reasons of harmonic mitigation? Does the EGC need to match the neutral in that case?

I see no issues with extending a 15A circuit with #12. As noted, it's common for dealing with voltage drop.

Originally Posted by pdh

What about cases where the neutral is oversized for reasons of harmonic mitigation? Does the EGC need to match the neutral in that case?

No, because the neutral is oversized in those cases for overload. The ground is more for fault clearing/shock protection, and is never going to see the extended overload like the neutral serving switched power supplies could.

What about cases where the neutral is oversized for reasons of harmonic mitigation? Does the EGC need to match the neutral in that case?

No, because the neutral is oversized in those cases for overload. The ground is more for fault clearing/shock protection, and is never going to see the extended overload like the neutral serving switched power supplies could.

250.122(B) doesn't seem to make that exception. Maybe it should? Or is there such an exception elsewhere?

Ground fault is typically resistive, but the inductive component comes into play, too. The NEC tables still apply- basically, you just calculate what current would cause 120V of voltage drop, and that's an upper limit for your fault current. The capacity of the source (transformer, generator, etc) comes into play, too, and there are some differential equations and all, but it's not hard to make conservative estimates. The important thing is that the ground wire be able to conduct current enough to trip the OCP before the wires explode, and that's what NEC is mandating.

Originally Posted by pdh

250.122(B) doesn't seem to make that exception. Maybe it should? Or is there such an exception elsewhere?

250.122(B) only refers to the ungrounded conductors; the neutral is implicitely excepted. None of the other items in 250.122 appear to apply to an oversized neutral, either.